Method for selectively chemically reducing CO2 to form CO

ABSTRACT

A method for selectively chemically reducing CO 2  to form CO includes providing a catalyst, and contacting H 2  and CO 2  with the catalyst to chemically reduce CO 2  to form CO. The catalyst includes a metal oxide having a chemical formula of Fe x Co y Mn (1−x−y) O z , in which 0.7≤x≤0.95, 0.01≤y≤0.25, and z is an oxidation coordination number.

TECHNICAL FIELD

The technical field relates to a process of selectively chemically reducing CO₂ to CO, and in particular it relates to a catalyst for the process.

BACKGROUND

Global warming makes the application of CO₂ attracts more attention. The conversion ratio of CO₂ into methane and methanol is not high due to thermodynamic limitation. However, if CO₂ is converted to CO first, and then converted to other hydrocarbon compounds (e.g. in an FT reaction), the conversion ratio can be dramatically enhanced. CO is a critical building block and a raw material used in chemical engineering, and its global production value is about 5,870 million U.S. dollars. CO is usually used to produce acetic acid, OXO alcohol, and the like. The acetic acid production in Taiwan is 750,000 tons per year, and the annual demand for CO exceeds hundreds of thousands of tons per year.

In general, CO₂ can be chemically reduced into CO using a reverse water gas shift reaction (e.g. H₂+CO₂→CO+H₂O) with a heat of reaction of 41.2 kJ/mole. As such, high temperature is beneficial to the reverse water gas shift reaction, and low temperature is beneficial to the water gas shift reaction. The water gas shift reaction (e.g. CO+H₂O→CO₂+H₂) has a heat of reaction of −41.2 kJ/mole. However, a methanation reaction also occurs between H₂ and CO₂. The methanation reaction (e.g. 4H₂+CO₂→CH₄+2H₂O) has a heat of reaction of −165 kJ/mole. As known from thermodynamics, low temperature is beneficial to the water gas shift reaction and the methanation reaction. The conventional method for chemically reducing CO₂ to CO is performed at a high temperature (e.g. 600° C.) with a low selectivity. For example, the product includes a high concentration of methane to be further separated, thereby increasing the cost of manufacturing CO.

Accordingly, a novel catalyst for selectively chemically reducing CO₂ to CO at low temperatures and lowering the CH₄ content in the CO product is called for.

SUMMARY

One embodiment of the disclosure provides a catalyst for selectively chemically reducing CO₂ to form CO, including a metal oxide having a chemical formula of Fe_(x)Co_(y)Mn_((1−x−y))O_(z), wherein 0.7≤x≤0.95, 0.01≤y≤0.25, and z is an oxidation coordination number.

One embodiment of the disclosure provides a method for selectively chemically reducing CO₂ to form CO, including providing the described catalyst; and contacting H₂ and CO₂ with the catalyst to chemically reduce CO₂ to form CO.

A detailed description is given in the following embodiments.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the disclosed embodiments. It will be apparent, however, that one or more embodiments may be practiced without these specific details.

One embodiment of the disclosure provides a catalyst for selectively chemically reducing CO₂ to form CO, including a metal oxide having a chemical formula of Fe_(x)Co_(y)Mn_((1−x−y))O_(z), wherein 0.7≤x≤0.95, 0.01≤y≤0.25, and z is an oxidation coordination number. If the Fe amount is too high or too low, the conversion efficiency of CO₂ will be insufficient, or the effect of selectively converting CO₂ to CO will be poor (e.g. the CH₄ content in the product will be too much). If the Co amount is too high or too low, the conversion efficiency of CO₂ will be insufficient, or the effect of selectively converting CO₂ to CO will be poor (e.g. the CH₄ content in the product will be too much). If the Mn amount is too high or too low, the conversion efficiency of CO₂ will be insufficient, or the effect of selectively converting CO₂ to CO will be poor (e.g. the CH₄ content in the product will be too much).

In some embodiments, the catalyst further includes a support, and the metal oxide is loaded on the support. For example, the support can be zirconium oxide, titanium oxide, silicon oxide, zeolite, aluminum oxide, another suitable support, or a combination thereof. In some embodiments, the metal oxide and the support have a weight ratio of 1:0.01 to 1:9. If the amount of the support is too high, the metal oxide amount will be too low to provide a sufficient conversion efficiency of CO₂.

One embodiment of the disclosure provides a method for selectively chemically reducing CO₂ to form CO, including providing the described catalyst; and contacting H₂ and CO₂ with the catalyst to chemically reduce CO₂ to form CO. In some embodiments, the step of contacting H₂ and CO₂ with the catalyst is performed at a temperature of 200° C. to 500° C. If the temperature of the step is too low, the conversion efficiency of CO₂ will be insufficient or even no conversion. If the temperature of the step is too high, the energy consumption will be increased to enhance the carbon emission. In some embodiments, the step of contacting H₂ and CO₂ with the catalyst is performed at a pressure of 1 bar to 40 bar. If the pressure of the step is too low, the throughput will be insufficient and therefore unsuitable for industrialization. If the pressure of the step is too high, the energy consumption will be increased to enhance the carbon emission. In some embodiments, H₂ and CO₂ have a molar ratio of 16:1 to 1:4. If the H₂ amount is too high, the selectivity of CO in the product will be lowered (e.g. the yield of CH₄ will be increased). If the H₂ amount is too low, CO₂ cannot be efficiently converted.

Note that in the metal oxide of the catalyst in the disclosure, the metal elements such as Fe, Co, and Mn cannot be arbitrarily replaced with another element. For example, if Co is replaced with Cu, the catalyst fails to chemically reduce CO₂ to CO at a low temperature (such as ≤500° C., or even ≤400° C.) with a high selectivity.

Accordingly, the catalyst in the disclosure may efficiently convert CO₂ at low temperatures, such as by selectively chemically reducing CO₂ to CO without forming CH₄. Compared to the conventional process of chemically reducing CO₂ to CO, the process in the disclosure is performed at a lower temperature (such as lower than 300° C.) with a higher selectivity of CO, thereby efficiently lowering the energy consumption of the process and reducing the separation cost (e.g. for separating CO and CH₄).

Below, exemplary embodiments will be described in detail so as to be easily realized by a person having ordinary knowledge in the art. The inventive concept may be embodied in various forms without being limited to the exemplary embodiments set forth herein. Descriptions of well-known parts are omitted for clarity, and like reference numerals refer to like elements throughout.

EXAMPLES Preparation Example 1 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z))

Iron nitrate (0.8 mol), cobalt nitrate (0.1 mol), and manganese nitrate (0.1 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. The neutralized solution was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z), and z was an oxidation coordination number.

Preparation Example 2 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z))

Cobalt nitrate (0.1 mol) and manganese nitrate (0.1 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. Iron hydroxide (0.8 mol) was added to the neutralized solution, and then stirred to be evenly mixed. The mixture was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z), and z was an oxidation coordination number.

Preparation Example 3 (Fe_(0.7)Co_(0.2)Mn_(0.1)O_(z))

Iron nitrate (0.7 mol), cobalt nitrate (0.2 mol), and manganese nitrate (0.1 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. The neutralized solution was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.7)CO_(0.2)Mn_(0.1)O_(z), and z was an oxidation coordination number.

Preparation Example 4 (Fe_(0.9)Co_(0.01)Mn_(0.09)O_(z))

Iron nitrate (0.9 mol), cobalt nitrate (0.01 mol), and manganese nitrate (0.09 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. The neutralized solution was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.9)Co_(0.01)Mn_(0.09)O_(z), and z was an oxidation coordination number.

Comparative Preparation Example 1 (Fe_(0.5)Co_(0.4)Mn_(0.1)O_(z))

Iron nitrate (0.5 mol), cobalt nitrate (0.4 mol), and manganese nitrate (0.1 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. The neutralized solution was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.5)Co_(0.4)Mn_(0.1)O_(z), and z was an oxidation coordination number.

Comparative Preparation Example 2 (Fe_(0.8)Co_(0.2)O_(z))

Cobalt nitrate (0.2 mol) was put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salt. The pH value of the solution was adjusted to be neutral. Iron hydroxide (0.8 mol) was added to the neutralized solution, and then stirred to be evenly mixed. The mixture was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.8)Co_(0.2)O_(z), and z was an oxidation coordination number.

Comparative Preparation Example 3 (Fe_(0.8)Cu_(0.1)Mn_(0.1)O_(z))

Iron nitrate (0.8 mol), copper nitrate (0.1 mol), and manganese nitrate (0.1 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. The neutralized solution was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.8)Cu_(0.1)Mn_(0.1)O_(z), and z was an oxidation coordination number.

Comparative Preparation Example 4 (Fe_(0.8)Mn_(0.2)O_(z))

Manganese nitrate (0.2 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salt. The pH value of the solution was adjusted to be neutral. Iron hydroxide (0.8 mol, Alfa) was added to the neutralized solution, and then stirred to be evenly mixed. The mixture was stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a metal oxide catalyst Fe_(0.8)Mn_(0.2)O_(z), and z was an oxidation coordination number.

Example 1

The metal oxide catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z) (20-30 mesh, 6 mL) in Preparation Example 1 was filled in a column. Hydrogen and carbon dioxide (GHSV was 800 to 1200 h⁻¹) were introduced through the column to contact the metal oxide catalyst. The column temperature was controlled to continuously react for 60 minutes, and the product composition was then verified by gas chromatography (with the carbonxen-1010 plot column), as shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (300° C.) of the metal oxide catalyst in Preparation Example 1 was close to the theoretical value, and the selectivity of CO in the product was very high.

Example 2

Example 2 was similar to Example 1, and the difference in Example 2 was the metal oxide catalyst being replaced with the metal oxide catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z) in Preparation Example 2. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (300° C.) of the metal oxide catalyst prepared by the different preparation condition was close to the theoretical value, and the selectivity of CO in the product was very high.

Example 3

Example 3 was similar to Example 1, and the difference in Example 3 was the metal oxide catalyst being replaced with the metal oxide catalyst Fe_(0.7)Co_(0.2)Mn_(0.1)O_(z) in Preparation Example 3. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (300° C.) of the metal oxide catalyst Fe_(0.7)Co_(0.2)Mn_(0.1)O_(z) was close to the theoretical value, and the selectivity of CO in the product was very high.

Example 4

Example 4 was similar to Example 1, and the difference in Example 4 was the metal oxide catalyst being replaced with the metal oxide catalyst Fe_(0.9)Co_(0.01)Mn_(0.09)O_(z) in Preparation Example 4. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (400° C.) of the metal oxide catalyst Fe_(0.9)Co_(0.01)Mn_(0.09)O_(z) was close to the theoretical value, and the selectivity of CO in the product was very high.

Comparative Example 1

Comparative Example 1 was similar to Example 1, and the difference in Comparative Example 1 was the metal oxide catalyst being replaced with the metal oxide catalyst Fe_(0.5)Co_(0.4)Mn_(0.1)O_(z) in Comparative Preparation Example 1. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (300° C.) of the metal oxide catalyst Fe_(0.5)Co_(0.4)Mn_(0.1)O_(z) was too low, and the selectivity of CO in the product was low. Accordingly, the metal oxide catalyst with too little Fe and too much Co was ineffective to selectively chemically reduce CO₂ to CO at a low temperature.

Comparative Example 2

Comparative Example 2 was similar to Example 1, and the difference in Comparative Example 2 was the metal oxide catalyst being replaced with the metal oxide catalyst Fe_(0.8)Co_(0.2)O_(z) in Comparative Preparation Example 2. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (300° C.) of the metal oxide catalyst Fe_(0.8)Co_(0.2)O_(z) was too low, and the selectivity of CO in the product was low. Accordingly, the metal oxide catalyst without Mn was ineffective to selectively chemically reduce CO₂ to CO at a low temperature.

Comparative Example 3

Comparative Example 3 was similar to Example 1, and the difference in Comparative Example 3 was the metal oxide catalyst being replaced with the metal oxide catalyst Fe_(0.8)Cu_(0.1)Mn_(0.1)O_(z) in Comparative Preparation Example 3. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (300° C.) of the metal oxide catalyst Fe_(0.8)Cu_(0.1)Mn_(0.1)O_(z) was too low, and the product still contained a little CH₄. Accordingly, if Co of the metal oxide catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z) was replaced with Cu, the metal oxide catalyst was ineffective to selectively chemically reduce CO₂ to CO at a low temperature.

Comparative Example 4

Comparative Example 4 was similar to Example 1, and the difference in Comparative Example 4 was the metal oxide catalyst being replaced with iron hydroxide. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (400° C.) of iron hydroxide was too low, and the product contained a lot of CH₄. Accordingly, iron hydroxide was ineffective to selectively chemically reduce CO₂ to CO at a low temperature.

Comparative Example 5

Comparative Example 5 was similar to Example 1, and the difference in Comparative Example 5 was the metal oxide catalyst being replaced with the metal oxide catalyst Fe_(0.8)Mn_(0.2)O_(z) in Comparative Preparation Example 4. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 1, and the results are shown in Table 1. As shown in Table 1, the CO₂ conversion ratio at a low temperature (400° C.) of the metal oxide catalyst Fe_(0.8)Mn_(0.2)O_(z) was too low. Accordingly, the metal oxide catalyst Fe_(0.8)Mn_(0.2)O_(z) was ineffective to selectively chemically reduce CO₂ to CO at a low temperature.

TABLE 1 Theoretical equilibrium conversion Reaction Reaction ratio CO₂ Selectivity temperature pressure (%, based conversion (%) Metal oxide catalyst H₂/CO (° C.) (bar) on CO₂) ratio (%) CO CH4 Example 1 Preparation 1 200 10 6.1  3.23 100    ND Example 1 250 9.6  7.16 100    ND (Fe_(0.8)Co_(0.1)M_(n0.1)O_(z)) 300 13.6  13.19 100    ND Example 2 Preparation 2 250  1 13.4  10.2  100    ND Example 2 300 19.0  19.0  100    ND (Fe_(0.8)Co_(0.1)M_(n0.1)O_(z)) Example 3 Preparation 2 200  1 8.6 5.4 100    ND Example 3 300 19.0  17.4  100    ND (Fe_(0.7)Co_(0.2)Mn_(0.1)O_(z)) 400 31.1  30.1  100    ND Example 4 Preparation 2 400  1 31.1  29.2  100    ND Example 4 500 42.5  41.9  100    ND Fe_(0.9)Co_(0.01)Mn_(0.09)O_(z) Comparative Comparative 1 250  1 9.6 3.8 85.4  14.6  Example 1 Preparation 300 13.6  10.9  32.4  67.6  Example 1 (Fe_(0.5)Co_(0.4)Mn_(0.1)O_(z)) Comparative Comparative 1 300  1 13.6   4.87 78.2  21.8  Example 2 Preparation  5 13.6   7.85 43.3  56.7  Example 2 10 13.6   9.77 26.3  73.7  (Fe_(0.8)Co_(0.2)O_(z)) Comparative Comparative 2 250  1 13.4  5.0 86.3  13.7  Example 3 Preparation 300 19.0  10.8  97.6  2.4 Example 3 (Fe_(0.8)Cu_(0.1)Mn_(0.1)O_(z)) Comparative Iron hydroxide 2 300  1 19.0  2.5 87.4  12.6  Example 4 400 31.1  4.3 87.9  12.1  Comparative Comparative 2 300  1 19.0  3.4 100    ND Example 5 Preparation 400 31.1  7.8 100    ND Example 4 (Fe_(0.8)Mn_(0.2)O_(z)) ND: Not detected

Preparation Example 5 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/ZrO₂)

Iron nitrate (0.8 mol), cobalt nitrate (0.1 mol), and manganese nitrate (0.1 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. Zirconium oxide powder (200 g) was added to the neutralized solution, stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a catalyst of metal oxide loaded on zirconium oxide (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/ZrO₂), and z was an oxidation coordination number. The metal oxide and zirconium oxide had a weight ratio of 1:2.2.

Preparation Example 6 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Al₂O₃)

Iron nitrate (0.4 mol), cobalt nitrate (0.05 mol), and manganese nitrate (0.05 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. Aluminum oxide powder (200 g) was added to the neutralized solution, stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a catalyst of metal oxide loaded on aluminum oxide (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Al₂O₃), and z was an oxidation coordination number. The metal oxide and aluminum oxide had a weight ratio of 1:4.5.

Preparation Example 7 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂)

Iron nitrate (0.2 mol), cobalt nitrate (0.025 mol), and manganese nitrate (0.025 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. Titanium oxide powder (200 g) was added to the neutralized solution, stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a catalyst of metal oxide loaded on titanium oxide (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂), and z was an oxidation coordination number. The metal oxide and titanium oxide had a weight ratio of 1:9.

Preparation Example 8 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Zeolite)

Iron nitrate (0.2 mol), cobalt nitrate (0.025 mol), and manganese nitrate (0.025 mol) were put into a beaker, and water (200 mL) was added into the beaker and stirred at room temperature to dissolve the salts. The pH value of the solution was adjusted to be neutral. Y-type Zeolite powder (200 g) was added to the neutralized solution, stirred for 2 hours, and then directly heated to 110° C. to be dried. The dried solid was heated to 450° C. to be sintered for 4 hours, thereby obtaining a catalyst of metal oxide loaded on zeolite (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Zeolite), and z was an oxidation coordination number. The metal oxide and zeolite had a weight ratio of 1:9.

Example 5

The catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/ZrO₂ (20-30 mesh, 3 mL) in Preparation Example 5 was filled in a column. Hydrogen and carbon dioxide (GHSV was 800 to 1200 h⁻¹) were introduced through the column to contact the catalyst. The column temperature was controlled to continuously react for 60 minutes, and the product composition was then verified by gas chromatography (with the carbonxen-1010 plot column), as shown in Table 2. As shown in Table 2, the CO₂ conversion ratio at a low temperature (400° C.) of the catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/ZrO₂ was close to the theoretical value, and the selectivity of CO in the product was very high.

Example 6

Example 6 was similar to Example 5, and the difference in Example 6 was the catalyst being replaced with the catalyst of the metal oxide loaded on aluminum oxide Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Al₂O₃ in Preparation Example 6. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 5, and the results are shown in Table 2. As shown in Table 2, the CO₂ conversion ratio at a low temperature (300° C.) of Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Al₂O₃ was close to the theoretical value, and the selectivity of CO in the product was high.

Example 7

Example 7 was similar to Example 5, and the difference in Example 7 was the catalyst being replaced with the catalyst of the metal oxide loaded on titanium oxide Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂ in Preparation Example 7. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 5, and the results are shown in Table 2. As shown in Table 2, the CO₂ conversion ratio at a low temperature (400° C.) of Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂ was close to the theoretical value, and the selectivity of CO in the product was very high. In addition, Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂ still had a high conversion ratio of CO₂ and a very high selectivity of CO in the product under a condition of high temperature and high pressure.

Example 8

Example 8 was similar to Example 5, and the difference in Example 8 was the catalyst being replaced with the catalyst of the metal oxide loaded on zeolite Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Zeolite in Preparation Example 8. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 5, and the results are shown in Table 2. As shown in Table 2, the CO₂ conversion ratio at a low temperature (300° C.) of Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Zeolite was close to the theoretical value, and the selectivity of CO in the product was high.

TABLE 2 Theoretical equilibrium conversion Reaction Reaction ratio CO₂ Selectivity temperature pressure (%, based conversion (%) Metal oxide catalyst H₂/CO (° C.) (bar) on CO₂) ratio (%) CO CH₄ Example 5 Preparation Example 5 2   250  1 13.4  6.5 100    ND (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/ZrO₂) 300 19.0  16.5  100    ND 350 25.0  23.8  100    ND 400 31.1  31.0  100    ND Example 6 Preparation Example 6  0.25 300  1 6.5 6.0 99.5  0.5 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/A1₂O₃) 1   13.6  11.4  98.3  1.7 2   19.0  16.7  94.4  5.6 Example 7 Preparation Example 7 2   300  1 19.0  4.4 100    ND (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂) 400  1 31.1  26.1  100    ND 400 30 31.1  29.8  100    ND 500  1 42.5  35.2  100    ND Example 8 Preparation Example 8 4   300  1 26.1  26.5  94.5  5.5 (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/Zeolite) 400 41.7  35.2  98.1  1.9 ND: Not detected

Example 9

Example 9 was similar to Example 5, and the difference in Example 9 was the gas hourly space velocity (GHSV) of H₂/CO being changed. The methods of controlling the reaction conditions and analyzing the product were similar to those in Example 5, and the results are shown in Table 3. As shown in Table 3, the CO₂ conversion ratio of a higher GHSV was lowered, but the CO production per unit amount of the catalyst of the high GHSV was higher than the CO production per unit amount of the catalyst of the low GHSV (CO/catalyst, kg/kg). The non-converted CO₂ and H₂ could be introduced through the catalyst column again to be selectively chemically reduced to CO. In addition, the selectivity of CO in the product was very high.

TABLE 3 Gas Theoretical hourly equilibrium space conversion Reaction Reaction velocity ratio CO₂ Selectivity temperature pressure (GHSV, (%, based conversion (%) Metal oxide catalyst H₂/CO (° C.) (bar) h⁻¹) on CO₂) ratio (%) CO CH₄ Preparation 2 400 1  800 31.1 26.5 100 ND Example 5  1600 26.0 100 ND (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/  3200 26.8 100 ND ZrO₂)  6400 25.8 100 ND 12800 24.7 100 ND 22400 23.4 100 ND ND: Not detected

Example 10

The catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂ (20-30 mesh, 100 mL) in Preparation Example 7 was filled in a column. Hydrogen and carbon dioxide (GHSV was 5000 h⁻¹) were introduced through the column to contact the catalyst. The column temperature was controlled to continuously react for 60 minutes, and the product composition was then verified by gas chromatography (with the carbonxen-1010 plot column), as shown in Table 4. As shown in Table 4, an H₂/CO ratio that is too high or a temperature that is too high would lower the selectivity of CO (e.g. the yield of CH₄ was increased).

TABLE 4 Theoretical equilibrium conversion Reaction Reaction ratio CO₂ temperature pressure (%, based conversion Product (%) Metal oxide catalyst H_(2/)CO (° C.) (bar) on CO₂) ratio (%) CO₂ H₂ CO H₂O CH₄ Preparation 2.0 500 13.8 42.5 41.0 19.7  53.0 13.7  13.7  ND Example 7 3.2 51.2 51.9 11.5  63.8 12.2  12.4  ND (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/ 4.0 55.4 55.5 8.9 68.9 11.1  11.1  0.1 TiO₂) 5.3 60.7 58.5 6.6 74.8 9.3 9.3 0.1 8.0 68.2 66.2 3.8 81.5 7.4 7.4 0.2 8.0 600 78.1 75.0 2.8 79.6 8.2 8.2 1.2 ND: Not detected

Example 11

The catalyst Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/TiO₂ (20-30 mesh, 100 mL) in Preparation Example 7 was filled in a column. Hydrogen and carbon dioxide (GHSV was 5000 h⁻¹) were introduced through the column to contact the catalyst. The column temperature was controlled to continuously react for 60 minutes, and the product composition was then verified by gas chromatography (with the carbonxen-1010 plot column), as shown in Table 5. As shown in Table 5, the product had high selectivity of CO at 400° C. The non-converted H₂ could be mixed with CO₂ and then introduced through the catalyst column again to form CO.

TABLE 5 Theoretical equilibrium conversion Reaction Reaction ratio CO₂ temperature pressure (%, based conversion Product (%) Metal oxide catalyst H₂/CO (° C.) (bar) on CO₂) ratio (%) CO₂ H₂ CO H₂O CH₄ Preparation 2.0 400 13.8 31.1 31   23.0  56.3 10.3  10.3  ND Example 7 3.2 38.1 36.6  15.1  67.5 8.7 8.7 ND (Fe_(0.8)Co_(0.1)Mn_(0.1)O_(z)/ 4.0 41.7 41.5  11.7  71.7 8.3 8.3 ND TiO₂) 5.3 46.4 46.5  8.5 76.7 7.4 7.4 ND 8.0 53.7 52.7  5.3 83.0 5.9 5.9 ND 16.0  65.9 65.7  2.0 90.3 3.9 3.9 ND ND: Not detected

It will be apparent to those skilled in the art that various modifications and variations can be made to the disclosed methods and materials. It is intended that the specification and examples be considered as exemplary only, with the true scope of the disclosure being indicated by the following claims and their equivalents. 

What is claimed is:
 1. A method for selectively chemically reducing CO₂ to form CO, comprising: providing a catalyst; and contacting H₂ and CO₂ with the catalyst to chemically reduce CO₂ to form CO, wherein the catalyst comprises a metal oxide having a chemical formula of Fe_(x)Co_(y)Mn_((1−x−y))O_(z), wherein 0.7≤0.95, 0.01≤y≤0.25, and z is an oxidation coordination number.
 2. The method as claimed in claim 1, wherein the step of contacting H₂ and CO₂ with the catalyst is performed at a temperature of 200° C. to 500° C.
 3. The method as claimed in claim 1, wherein the step of contacting H₂ and CO₂ with the catalyst is performed at a pressure of 1 bar to 40 bar.
 4. The method as claimed in claim 1, wherein H₂ and CO₂ have a molar ratio of 16:1 to 1:4. 